1. Field of the Invention
The present invention relates to an image processing device that performs image processing of data for a target image having color information for each pixel and outputs the thus-processed data for an output image, and an image forming apparatus that performs image forming of the output image data.
2. Description of the Related Art
An image forming apparatus such as a copier, a facsimile, and a printer receives data described in a page description language (PDL) from a personal computer, etc. and processes the data in an image processing device. An example of such an image processing device is described in Japanese Patent Application Laid-open No. H9-168086. An image processing device generally performs half-tone processing such as gradation processing and dithering based on an error diffusion technique and a dither technique on input data (color image data) described in a PDL. This is for the following reason. In image data described in a PDL, color information is expressed in a tone value for each pixel. Furthermore, the color information is provided for each of the three primary colors (RGB or CMY). It is therefore difficult to individually and subtly control the amount of coloring material (e.g., ink or toner) consumed for each pixel in a color image to be formed by the image forming apparatus. This makes it difficult to perform half-tone processing in pixel units. Hence, in general image forming processes, color image data, in which half-tone is represented in pixel units, is converted through dithering into image data, in which half-tone is represented in units of an area made of a plurality of pixels.
By performing half-tone processing on color image data having color information for each pixel, the amount of data can be substantially reduced. To give a specific example, when each of the RGB (or CMY) colors is represented using 256 tones, each pixel is represented in a massive number of colors of 16,777,216 (256×256×256) colors. However, in this case, because 24 bits of information is required as color information for each pixel, the amount of image data to be processed will be extremely large, which is undesirable. Contrastingly, image data having undergone half-tone processing requires only indication as to whether each of colors (for example, CMY) is used for each pixel in the image to be outputted. It is therefore possible to substantially reduce the amount of data.
As described above, the amount of image data having undergone half-tone processing is small. Hence, by performing such image processing on image data, the amount of memory required for the image processing can be substantial reduced as compared to image processing carried out on image data the amount of which prior to being subjected to half-tone processing is large. However, it is possible that carrying out image processing on image data that has been subjected to half-tone processing is not appropriate for some type of the image processing. There can be even cases where carrying out image processing on image data having undergone half-tone processing results in failure.
As a specific example, image processing that draws a base image and thereafter performs masking such as watermarking on a predetermined region of the base image to obtain a final image will be described. Meanwhile, image data that has been subjected to half-tone processing is represented with a half-tone using a surface area ratio for each color of a pixel group comprised of a plurality of pixels. Accordingly, it is not possible to directly specify the color of each pixel of the base image from the image data for the image to be outputted. When, for example, image data in which commands and the like for carrying out image processing are described in a PDL is inputted, it is necessary to carry out the image processing on the image data prior to perform half-tone processing of the image data (image data having color information for each pixel). Examples of image processing that has to be carried out or is appropriate to be carried out on image data prior to being subjected to half-tone processing are not limited to masking.
As described above, because image data that has not yet been subjected to half-tone processing is large in the amount of data, a memory capacity required to perform image processing on such image data is increased. To this end, in recent years, resolution (for example, 1,200 dots per inch (dpi)) of image data (input data) to be subjected to image processing is temporarily dropped down to a lower resolution (for example, 600 dpi) in order to keep the memory capacity required for this image processing small. After the image data with the reduced resolution (low-resolution image data) is subjected to image processing, the resolution of this image data is increased to the resolution (for example, 1,200 dpi) of an image to be outputted. The memory capacity can therefore be reduced by an amount corresponding to the decrease in the resolution.
However, when the resolution has been dropped in this manner in the image processing, even if the resolution of the image data is attempted to be increased to the resolution of the image to be outputted after the image processing, the actual resolution remains to be low. Typically, the influence of the low resolution on the quality of pictorial images such as photographs, drawings, and patterns is comparatively small when viewed with eyes. In contrast, reducing the resolution to a small value is likely to cause the images to have apparent jaggedness, thereby substantially reducing the quality of images of characters such as Japanese characters, Chinese characters, alphabetic characters, and symbols when viewed with eyes in such a manner. Jaggy-smoothing technology that smoothes jaggy has been known, and an example of the technology is described in Japanese Patent Application Laid-open No. H9-168086. However, the known jaggy-smoothing technology has failed to sufficiently increase the quality of images of small-sized characters, etc. when viewed with eyes.